Processes for the liquefaction of natural gas are principally of two main types. The classical method is the "cascade" refrigeration cycle, which is theoretically quite efficient. It provides a series of refrigerants selected so as to provide only small temperature differences between the refrigeration system and the natural gas being cooled. By using a sequence of refrigerants it is possible to cool natural gas from ambient temperature as received from wells or pipelines down to about -259.degree. F., typical of LNG. Such a system is typified by U.S. Pat. No. 3,020,723. A principal characteristic of the cascade refrigeration cycle is that the coldest refrigerant discharges heat received from the natural gas into the next warmer refrigerant, which in turn discharges that heat along with the additional heat it receives from the natural gas to its next warmer refrigerant. This cascading is continued until finally all the heat is rejected to the ambient temperature surroundings by the warmest refrigerant.
While the cascade refrigeration system is efficient thermodynamically, it is mechanically complex since it requires many compressors, which is inherently undesirable. In addition, the efficiency of the liquefaction process is affected by the lower mechanical efficiency of smaller compressors. To avoid multiple compressors, other plants have used multi-component refrigerant (MCR) cycles which approximate the cascade system by using a multi-component refrigerant tailored to provide a cooling curve which approximates that of the liquefaction of natural gas but, having only a single refrigerant, can use a single compressor. See U.S. Pat. No. 3,593,535 for a typical MCR liquefaction process. The MCR refrigerant is composed of components having boiling points ranging from nitrogen to C.sub.6 hydrocarbons. As an approximation of the operation of the cascade system, it is characteristic of MCR refrigeration cycles for a refrigerant stream to be removed from the cooling process and the heaviest components separated and then flashed for use as a refrigerant, and returned to the compressor. Thus, while the refrigerant is originally mixed, it is separated by successive flashing so as to provide the staging of the refrigeration temperatures which are needed to cool and condense natural gas. Such MCR cycles, while lacking the thermodynamic efficiency of the cascade cycle, cost less to build and operate since they require simpler equipment and permit the utilization of large-scale compressors which are inherently less expensive and more efficient mechanically than the multiple smaller units required for the cascade system.
A recent development and modification of the MCR refrigerant cycle is what has been called a "cold-suction" cyle. Such a system might be considered to be a hybrid between the cascade and MCR cycles in that the cold-suction cycle operates with cooling supplied from a separate refrigeration system at the warm end of the LNG liquefaction plant while still using an MCR cycle at the cold end of the plant. The MCR refrigerant in a cold-suction cycle does not warm up to the temperature of the incoming feed but instead is sent to the compressor suction while it is still cold and retains available refrigeration. Since the compressor horsepower is directly proportional to the absolute temperature at the suction of the compressor, substantial savings in compression horsepower can be achieved. A number of variants are possible and typical of these are described in applications Ser. Nos. 309,341 and 304,276 which are assigned to the same assignee as is the present application. The separate refrigeration may be a propane refrigeration system or alternatively, may be provided by a secondary loop established within the MCR refrigeration system. Both such refrigeration schemes are disclosed in the copending applications previously cited. A cold-suction cycle has two main disadvantages. First, while compression costs are reduced, additional heat exchangers and utilities are required, and secondly, supplying refrigeration to the warm end of the LNG plant from a separate refrigeration system is inherently less efficient thermodynamically than using the returning cold MCR refrigerant. In addition, additional complexity is introduced into the operation of the MCR refrigeration system when such extraneous refrigeration is supplied to it.
While the "cold-suction" refrigeration system has inherent cost advantages over the pure MCR cycle, its disadvantages suggest further improvement is required to reach an optimum refrigeration system. Overall, it may be said that the objective in the design of any such liquefaction process is to optimize capital costs and utilities while minimizing complexity in order to reduce manning and potential upsets. Such an improved system has been discovered and is the subject of the present application.